[en] The multicoloured Asian ladybird Harmonia axyridis Pallas (Coleoptera: Coccinellidae), previously introduced as a biological control agent against aphids, is now frequently considered as an intraguild predator, consuming other aphid natural enemies. Interactions between this exotic ladybird and other aphidophagous species present in Belgian agro-ecosystems such as Coccinella septempunctata, Episyrphus balteatus and Chrysoperla carnea are asymmetric to the benefit of H. axyridis.
This study focuses on the survival rate of three aphidophagous species Harmonia axyridis (Ha), Coccinella septempunctata (C7) and Episyrphus balteatus (Eb) placed in cages in biological fields of potato and sugar beet. During June and July, hermetic cages were disposed on the crops with 10 larvae (second stage) of each aphidophagous. Four types of cages were observed (1) Ha+Eb+C7+ aphids, (2) Ha+Eb+C7, (3) Eb+C7+ aphids and (4) Eb+C7. Each combination was repeated three times.
In sugar beet fields, E. balteatus was the species with the highest mortality rate. The two coccinellid species had a better survival rate than E. balteatus. All aphidophagous species had higher mortality rates in absence of aphids. In potato, when aphids were present in excess, the survival rates of all aphidophagous insects were lower than in sugar beet probably because A. fabae is less adapted to the potato morphology. Nevertheless, we have observed in potato no mortality of C. 7-punctata during all its development cycle. This suggests that (1) C. 7-punctata may use aphids as food sources more efficiently than H. axyridis and (2) that interactions between aphidophagous could be less frequent (difficulty of mobility). In potato and sugar beet, H. axyridis had the longest development cycle, being still at the larval stage when C. 7-punctata and E. balteatus reached the pupae stage. We can suppose that pupae of E. balteatus and C. 7-punctata were an alternative food (intraguild prey) to H. axyridis when aphids were lacking. Pupae are immobile and therefore more sensitive to the attack of predators.